1. Technical Field
The present invention relates to an image display apparatus.
2. Related Art
It is hoped that projectors and other image display apparatus can display an image at higher resolution. When an image formed by using a light modulator, such as liquid crystal light valve, is displayed, the number of pixels of the image displayed on a screen or any other surface is typically equal to the number of pixels of the light modulator. Increasing the resolution of the light modulator increases the resolution of the displayed image but results in significant increase in manufacturing cost.
To produce a high-resolution displayed image without increasing the resolution of the light modulator, it is conceivable to increase the number of light modulators. Forming images by using a plurality of light modulators and projecting the thus formed images in such a way that the position of each pixel of one image is shifted from those of the other images allow the total number of pixels on the screen to be increased. In this method, however, since the number of light modulators is increased, the cost increases accordingly. As a technique for solving the inconvenience described above, instead of increasing the number of light modulating devices to form a plurality of images, a method for forming a plurality of images in a time division manner by using a single light modulator has been proposed (JP-A-11-298829 and JP-A-2005-91519, for example).
In the image display apparatus described in JP-A-11-298829 and JP-A-2005-91519, image light formed by a light modulator in a time division manner is projected through a flat-plate prism. The flat-plate prism is inclined to the direction in which the image light is incident. The image light incident on the flat-plate prism exits therethrough with the optical path of the image light shifted in parallel. The amount of shift of the optical path before the light is incident on the flat-plate prism from the optical path after the light is incident on the flat-plate prism is controlled in synchronization with image formation. As a method for changing the amount of shift with time, JP-A-11-298829 describes the following first to third methods, and JP-A-2005-91519 describes a fourth method.
In a first method, the inclination angle of a flat-plate prism is changed with time. In a second method, a flat-plate prism formed of portions that produce different amounts of refraction is rotated to change with time the amount of refraction produced in the portion through which image light passes. In a third method, a flat-plate prism is made of a nonlinear optical crystal the refractive index of which can be variably controlled by applying an electric field, and the applied electric field is changed with time. In a fourth method, a light blocker is provided between the portions that produce different amounts of refraction in the second method.
The techniques described in JP-A-11-298829 and JP-A-2005-91519 have the following problems:
In the first, second, and fourth methods, the flat-plate prism could vibrate when spatially displaced. If the flat-plate prism vibrates, the amount of shift of the optical path changes unexpectedly, which makes it difficult to control the amount of shift with high precision, resulting in a decrease in the quality of a displayed image. An attempt to synchronize the displacement of the flat-plate prism with image formation in a spatial light modulator with high precision makes a mechanism for spatially moving the flat-plate prism complicated. Further, the vibration of the flat-plate prism could produce noise and shorten the lifetime thereof.
In the first method, since images are displayed even while the pixels are shifted, the image could be blurred. In the second method, since the pixels in the vicinity of the boundary between the portions that produce different amounts of refraction are separated, and the pixel shift timing on one end side of a displayed image differs from the pixel shift timing on the other end side of the displayed image, for example, the quality of the displayed image could decrease. Although employing the fourth method can prevent the pixels from being separated, the problem of the difference in the pixel shift timing in a displayed image is still unsolved.
In the third method, the size of the nonlinear optical crystal needs to be greater than or equal to the spot size of incident image light. Applying an electric field strong enough to ensure the amount of shift necessary to provide a sense of high resolution to the thus sized nonlinear optical crystal requires a voltage higher than those for driving a light modulator and other components, resulting in an increase in voltage required to drive the entire image display apparatus.
In the third method, using the Kerr effect disadvantageously increases the cost of the nonlinear optical crystal, resulting in loss of superiority over a method of increasing the number of light modulators. On the other hand, using the Pockels effect results in a decrease in light usage efficiency because the visible light transmittance of the nonlinear optical crystal decreases, necessity of controlling the state of the nonlinear optical crystal, and other inconveniences. As described above, it is not realistic to variably control the optical path of image light by using a nonlinear optical crystal.